The State of the Indie Business

[inverse]

I think you're complicating this as much as the ODE docs guys did.

Sorry, wasn't quite sure how detailed an answer you were expecting.

When I worked at Stainless we had our own physics engine, and to set the centre of gravity you just gave it a vector offset from its graphics origin. Not sure what else is needed or why this other stuff exists at all tbh. (Nope, not a mathematician)

The other stuff is there mostly because... well, think of a long board/stick/ruler. Grab one end and twist it along the length, it's pretty easy to spin. Grab it in the middle and spin it like a propeller and it's more difficult, even though the total mass is the same. The inertia tensor makes the object act like that, instead of just like a ball.

If all you want to do is have the rigid body's center of mass not be at the render model origin, you'll just need to compensate for that offset when you get the transform out of the rigid body. That doesn't involve the inertia tensor at all.

And for most purposes, just computing the inertia tensor from the model's bounding box is probably sufficient.

[Applewood]

Right, thanks for the explanation on why the tensor exists. That single paragraph was far more intuitive that anything I've read so far!

Regarding the CoG, is this going to work? If I have a box lying on the ground with it's CoG a lot below the ground, and apply a torque to it, it wants to rock on the ground but according to the pendulum action it would have from the CoG, not from the ground.

Bascially, the centre of mass needs to be set at an arbitrary point outside of its "visible mass". It's not so much the graphics, but the position relative to the bounding object. The tensor seems to be the only way to add this extra parameter but I can't work out how.

And I really don't get how I seem to be the only guy thinking this is a bit odd so I hope I'm missing something. Couldn't find anything online. Am assuming my question has the same effect it does when a layman asks a mathematician anything - that look on their face as if you just shit in their mouth. Or is that just the two experts I used to know?!

[inverse]

Regarding the CoG, is this going to work? If I have a box lying on the ground with it's CoG a lot below the ground, and apply a torque to it, it wants to rock on the ground but according to the pendulum action it would have from the CoG, not from the ground.

Bascially, the centre of mass needs to be set at an arbitrary point outside of its "visible mass". It's not so much the graphics, but the position relative to the bounding object. The tensor seems to be the only way to add this extra parameter but I can't work out how.

Yes, that will probably work to make the box rock, though it might look kind of odd. I've done the same thing with vehicles to make them feel more stable, though that's usually just pushing the CoG low, not right outside the collision volume. That can get weird.

Think of it this way: If the box is just floating in space with no gravity, when you push on the box, it's going to spin around its center of mass regardless of the inertia tensor. The inertia tensor defines how fast it will be spinning after a given force.

Basically:
Center of gravity = Center of spin
Inertia tensor = How fast it spins

If the center of mass is outside the visible box, it will look like you're spinning a box around on the end of a string, without the string. When you put a floor in the way, you end up with something maybe more like a boat floating on water - there's a bunch of mass pulling things around below the surface.

Now, if the box with the CoG at the visible center isn't acting like you expect, it might be because the inertia tensor isn't set up quite right, so you're seeing the box behave as though it has a different shape or mass (i.e. what looks like a large heavy box, acting like a small light box).

You might want to play with the friction/restitution values as well, making the collision more or less bouncy might help give the effect you're looking for.

[Applewood]

Sorry for being dumb, but I'm just not getting this. If the graphics for the car and the collision geometry for the car are both running along the ground, what bit of magic do you do to push the CoG lower please?

I'm hoping this is gonna snap shortly, I'm not that stupid but this just isn't going in. Thanks!

[inverse]

Moving the CoG is basically the same thing as moving the origin of the collision geometry.

It looks like dGeomSetOffsetPosition(...) is what you would use in ODE to do that.

To get the render geometry to match, you need to move the origin of the render model to match the new CoG:

render_transform = rigid_body_transform * inverse(CoG_offset)
(assuming you multiply your vertex on the right, otherwise reverse the multiplication order)

I hope that does what you're looking for...

[Applewood]

I think that's exactly the monkey, thanks very much indeed!

[lennard]

"Free games that suddenly ask for money are scams!"

That stuff just drives me nuts and, frankly, I agree with Applewood that you just can't look at the 1's because the lowest common denominator review info. is just noise.

[jcottier]

"iOS 5 update take too long" --> 1 star
This is one is pissing me so much that I just delay the iOS5 update a bit more.

JC

[Son of Bryce]

Applewood, is there any difference between publishing on Windows RT and Windows Phone?

[Applewood]

Nothing substantive, but it doesn't "just work" either.

[Bram]

Applewood,

Sorry for the late reply.

You need to define the body position at the centre of mass, and never let them deviate.
Mind you, you can still distribute collision geometry around this centre as you wish, or redistribute mass.
E.g. in a sphere shape, box shape, ring shape, whatever.

But the logical position of an object should be the centre of mass for the body.

E.g. a trick to keep a car from rolling over: define the centre of mass (car pos) at the bottom, or even below its chassis.
The collision geometry will all be above this point, but that is fine.

[Applewood]

Yep, that makes sense. Thanks muchly.

[Jamie W]

"iOS 5 update take too long" --> 1 star
This is one is pissing me so much that I just delay the iOS5 update a bit more.

JC

That's an improvement over Android. I got rated down because the player has to pay (single IAP) to progress past a certain point in the game, and only 3 stars because you can download it free on some pirate site!

WTF!? User reviews giving less stars because you're asking them to pay.

[Applewood]

I got one of those just recently. One star because Android Backup doesn't work on his phone.

We are over populated if we can support people this crap.

[Bram]

I got one of those just recently. One star because Android Backup doesn't work on his phone.

Actually... I started to appreciate the really really bad reviews, because of the sheer humour value.
So I collect the worst of the worst.
I've seen some devs actually print them out on a custom coffee mug.

Here's a gem I dug up on my game:


Now, this is pure comedy gold! I love 'em :-)
It brings out my evil laugh 'Mwoah ha ha ha!"

[Applewood]

Love it. I'd be tempted to celebrate these with some comedy mugs, but it might encourage more of them. LOL

[BantamCityGames]

hopefully his "lawyers parents" aren't assassins

[lennard]

The lawyers parents line is awesome - you should send a couple of bucks to somebody to make you a coffee mug with that quote.

[Jamie W]

Yeah, there's definitely comedy value in these, esp. if they're swamped out by mostly positive reviews. Here's one of my favourites (Retro Racing on iOS):

[Jack Norton]

Lol I love the mention of "Bill Gates". WTF really

[Nutter2000]

Hahaha they're fantastic! :-D

Definitely love the coffee mug comment!

[jcottier]

Whoa, those reviews are awesome.

JC

[jcottier]

I did find this so funny that I posted a thread related to this topic in the Touch Arcade forum:

http://forums.toucharcade.com/showth...68#post2641068

You guys should definitely post there with your quotes, who knows we might get notice and earn some extra traffic to our games.

JC

[BantamCityGames]

I wish I had a recent product and enough customers to even have quotes